Solder resists are placed on a printed wiring board to protect selected areas of the conductive pattern from the action of molten solder. The solder resist pattern usually is applied to leave the terminal areas or lands exposed to the solder and to cover the rest of the conductors.
One type is the screen printed solder resists which are applied by by forcing a liquid resist coating through a stencil screen. The screen printed image is cured to a solid, thermoset coating by heat or ultraviolet energy.
Photoimaged resists can resolve finer image features and produce more tightly defined patterns than screen printed resists, and they are preferred for printed wiring boards with high density or fine line patterns.
A common type of photoresist is the dry film photoresist which is applied to the printed wiring board by hot roll lamination. The resist coated board is exposed to ultraviolet light through a photographic negative and developed in a solvent to remove the unexposed portions of the resist.
Another group of photoimageable, solder resists are applied to the printed wiring board as liquids by roller coating, curtain coating or blank screen printing without a stencil. The application method depends of the viscosity of the liquid and the chemistry of the resist. Roller coating and curtain coating have a disadvantage in that special equipment is required which is not generally available in printed wiring board manufacturing facilities.
The shelf life of the resist liquids also depends on the chemistry of the resists. Some liquid, photoimageable, solder resists cannot be supplied as a one part system because the one part system with those chemical components would have a shelf life of a few days or weeks. These are supplied as a two part system that the user mixes together before use. Two part systems must be mixed in house which creates quality control and storage problems for the user.
The exposure method for photoimaging the liquid, solder resists varies. Some are dried and then contact printed by exposure to ultraviolet light through a photographic negative. However, the low molecular weight resins in the dried, unexposed resist tend to get tacky during the exposure as the exposure equipment heats up in use. This causes the dried, resist film to stick to the negative. Other liquid, resist coatings are exposed wet with the negative off contact. These have the disadvantage of requiring a special off-contact printer which is a unique piece of equipment not generally used in the industry.
In another system, which also requires a specialized coating and exposure equipment, the negative is coated with resist which is partially cured to a gel. The printed wiring board is coated with liquid resist, placed in contact with the gel coated negative and exposed to ultraviolet light. The liquid resist on the board and the gelled resist on the negative adhere together and are solidified by the exposure.
The generally preferred method for applying liquid, photoimageable, solder resists is blank screen printing. Most of the solder resists applied by blank screen printing are two part systems which has restricted their use.
The photoimaged resists are developed after exposure to ultraviolet light by removing the unexposed portions of the resist with solvents. Organic or aqueous solvent developers are used depending on the chemistry of the resist. Aqueous developers are easier to waste treat and most users prefer resists that develop in aqueous sodium carbonate solutions.
The liquid, photoimageable, solder resists require a final cure after development. Standard cures are a very high energy ultraviolet exposure of 25-50 mJ/mm.sup.2 plus a thermal cure of 30-60 minutes at 140.degree.-160.degree. C.